What are the flutter requirements in aircraft design?

Flutter is a dynamic aeroelastic instability that can cause structural failure. Requirements: FAR 25.629 requires aircraft to be free from flutter throughout the flight envelope to VD/MD (dive speed/Mach) plus margins. Analysis approach: Ground Vibration Testing (GVT) to characterize structural modes; Unsteady aerodynamic models (doublet lattice, CFD); Frequency-domain flutter analysis (V-g method); and Time-domain analysis for nonlinear effects. Design considerations: Stiffness requirements for control surfaces, Mass balance of control surfaces (weight forward of hinge), Structural damping requirements, Fuel loading effects on wing modes, and External stores effects (military). Testing: Flight flutter testing with excitation and response measurement, Envelope expansion with flutter margin verification. Prevention: Adequate structural stiffness, Proper mass distribution, Control surface balance, and Avoiding resonance conditions. Flutter margins (typically 15-20% above VD) ensure safety with manufacturing variations and damage.